Hi I just wonder why fast lens or large aperture lens is the lens to get when the shapness of IQ is somewhere in the smaller aperture setting.

I know that fast lens can take better pics in a low light situation but its not as sharp as if i m using a smaller aperture lens with a flash.

I just bought a Tamron 17-50 f2.8 VC for my D90, and from most of the reviews i read its one of the sharpest lens even wide open. I really dont know when I should be using the aperture set to f2.8, coz I felt that its not as sharp as I m using a flash. I m comparing this when I took pics of my kids in my house.

Wide aperture is useful for shallow depth of field, i.e blurring out the background in your photos.

For moving subjects/sports the wide aperture is useful to prevent motion blur as the subject moves/runs.

Each full f/stop i.e. f/5.6 to f/4 allows double the amount of light into the camera. With more light allowed in the shorter the time the shutter needs to be open to get a balanced exposure, and this faster shutter speed means you are taking a shorter snapshot of the action. In this shorter time the subject/player will have moved less.

The sharpness depends on the quality of the lens too. A good lens will still be plenty sharp wide open or close to it. Usually these are the more expensive lenses too, naturally. As you say, it's a balancing act: you're trying to get a sharp subject with lighting conditions and activity (or lack thereof) that may require a bigger aperture to even get it in the first place. You might be willing to give up a tony bit of sharpness to get that shot in low light.

But we aren't talking about blurry subjects with a wide aperture, merely less than perfect sharpness. The difference between your slens' sharpenss wide open and at f/8 or f/11 is usually very subtle. More expensive, better lenses simply make it more subtle. I think it's important to remember that a lot of highly informative and technical reviews highlight very fine grain differences in these qualities where you won't notice or miss them in the so-called "real" world. I think Camera Labs is good at putting this kind of thing in context in their reviews. To take a shot with a wide open aperture, otherwise correctly focused and exposed, versus the same shot with the aperture stopped down (balancing shutter and ISO) simply yields a different kind of image but not a truly blurry one, not with a decent lens anyway.

On the other end of things, people worry a lot about diffraction errors at the smallest apertures, but this too I think is a bit exaggerated by the nature of the technical "dog-sniffing" as I call it that these review sites need to do. I just had a friend buy himself a new DSLR and was almost paralyzed in fear of making the wrong decision about his camera choice because he couldn't see the forest from the trees and put these things in perspective (pardon the pun).